(1) Field of the Invention
The invention relates to a loudspeaker enclosure.
(2) Description of the Prior Art
The primary purpose of a loudspeaker enclosure is to prevent an acoustical short circuit. This acoustical short circuit can occur, for instance, when the air in front of the loudspeaker membrane is compressed while the air behind the loudspeaker membrane expands during forward motion of the speaker. Thus, the acoustical oscillations from the front of the membrane are in a phase which is in opposition to the phase of the acoustic oscillations from the rear of the membrane. If the acoustical oscillations from the rear of the membrane interfere with the acoustical oscillations from the front of the membrane, then the oscillations may tend to cancel each other and no sound is produced. However, the effect of the acoustical short circuit is only noticeable at low frequencies. The effect of the acoustical short circuit is negligible at high frequencies at which the sound is practically unidirectional.
By placing the loudspeakers in an enclosure, acoustic oscillations from the front of the loudspeaker membrane are prevented from traveling to the rear of the loudspeaker membrane. Acoustical short circuiting is thus reduced or avoided.
However, the use of loudspeaker enclosures creates other undesirable effects. One of these is the natural resonance frequency of the enclosure. To a great extent, this resonance is determined by the size of the enclosure and the material from which the enclosure is made. The elimination of the enclosure resonance is essential in obtaining a sound reproduction free of distortion. In order to eliminate the resonance, the enclosure should be constructed of a material with high intrinsic damping. The enclosure must also possess a certain density.
The sound radiation produced by resonance of the enclosure walls is determined by different factors: If the frequency is low, sound radiation depends upon the flexural strength of the enclosure walls. If the frequency is high, the sound radiation depends upon the material which covers the enclosure walls. If the frequency is in the area of the various modes of resonance of the enclosure, then the sound radiation depends upon the damping of the enclosure walls.
In order to meet the conditions listed above, it has been suggested that speaker enclosures be made of spruce, fir or birch wood, with double walls separated by a hollow space. The hollow space would then be filled with fine dry sand or with rigid expanded polyurethane (Klinger, Lautsprecher und Lautsprechergehause fur HiFi, 7. Auflage, 1975, S. 74). The construction of loudspeaker enclosures of wood is, however, relatively expensive since the walls have to be relatively thick and since the exterior of the enclosure must be finished.
One loudspeaker enclosure presently manufactured uses at least two layers of corrugated cardboard with the grooves placed vertically (German Patent Application No. 23 04 711). The surface treatment of these loudspeakers is inherently quite difficult. The same is true for the loudspeaker enclosure disclosed in U.S. Pat. No. 3,848,696 wherein one supporting layer is connected with at least two corrugated layers of foil.
In another loudspeaker cabinet, the enclosure is either partially or completely made out of polystyrene (German Utility Model No. 18 21 346). In order to diminish the resonance, the enclosure walls are rounded. Furthermore, the entire enclosure is covered with washable plastic. The disadvantage of this loudspeaker cabinet is that the enclosures resonates.
In order to avoid the drawbacks of plastic enclosures, a loudspeaker cabinet has been proposed which is completely filled with plastic foam. It would conform to the shape of the loudspeaker(s) in the rear and the exterior sides could be covered with an impact-resisting material (German Patent Application No. 15 12 746; see also German Utility Model No. 76 00 131). However, the application of the impact resisting material to the plastic foam would involve complicated processes.
Constructing enclosures as hollow frame shells so that plastic foam can be blown into the shells has the disadvantage that the resonance of the enclosure would still be too high.
One object of the present invention is to provide a loudspeaker enclosure having relatively thin walls and which may be easily manufactured.
It is another object of the present invention to provide a loudspeaker enclosure in which enclosure wall resonance is effectively damped so that only minor sound radiation eminates from the loudspeaker enclosure.
It is another object of the present invention to provide a loudspeaker enclosure that is particularly well suited for relatively small loudspeakers.
It is another object of the present invention to allow for relatively simple fabrication of the loudspeaker enclosure and to allow for simple installation of the front and rear panels of a loudspeaker enclosure.